+/**
+ * Treat the bitset as a number and subtract 1.
+ * @param bs The bitset.
+ * @return The same bitset.
+ */
+static INLINE void bitset_minus1(bitset_t *bs)
+{
+#define _SH (sizeof(bitset_unit_t) * 8 - 1)
+
+ bitset_pos_t i;
+
+ for(i = 0; i < bs->units; ++i) {
+ bitset_unit_t unit = bs->data[i];
+ bitset_unit_t um1 = unit - 1;
+
+ bs->data[i] = um1;
+
+ if(((unit >> _SH) ^ (um1 >> _SH)) == 0)
+ break;
+ }
+#undef _SH
+}
+